Spent nuclear fuels, which are high level waste (HLW), have been generated with lots of radiation and heat from nuclear power plants. According to statistics information in Korea, in temporal storage, the number of spent fuel assemblies is about 11,000, and predicted number of spent fuels is about 71,000 assemblies until 2030. It is predicted that lots of spent fuel will be generated in the future; therefore, disposal and storage problems of spent fuels have gotten increased. Generally, spent fuel storage is designed to improve storage efficiency in limited area. To design spent fuel storages, criticality safety, radiation shielding, heating removal and structural integrity should be considered. Especially, criticality safety is an important safety guideline to prevent criticality accident. To satisfy criticality safety criteria and design dense storage, neutron absorber has been used. Existing neutron absorbers such as BORAL™, NeutroSorb™, and the others, which are metal-based materials, have been used. These neutron absorbers have high neutron absorber ability and mechanical strength. However, the cost of these absorbers is high. Also, it can be easily corroded with salt water (especially aluminum-based absorber). In previous study [1], to solve the disadvantages of existing neutron absorber, newly-designed neutron absorber based on vitrified form is proposed. The vitrified form which is one of radioactive wastes is generated from vitrified form facility. And then, vitrified form is reused as criticality control material in storage facility. Using the neutron absorber and storage concept, it can simultaneously store both spent fuel and vitrified form in storage. Therefore, it can increase the storage efficiency of the radioactive wastes as well as to reduce the disposal cost.

The vitrified form technique is a method to eternally prevent radioactive nuclides composed in glass structure into environment. Thus, the radioactive nuclides in vitrification form are enclosed in physically and chemically strong glass formation. Boron is contained in vitrified forms to increase thermal resistance and mechanical strength. Boron has high absorption cross section; therefore, it can be used for criticality control material in nuclear-related facilities. In addition, vitrified form has high corrosion resistance and good thermal property. The brief description on the properties of vitrified form is given in Table 1.

In previous studies [1, 2], the applicability of vitrified form as neutron absorber was verified. For the application of the vitrified forms as neutron absorber, the neutron absorption ability should be properly verified. To verify their neutron absorption ability, some problems can be occurred. Vitrified form is radioactive waste; therefore, it is difficult to perform chemical analysis. Another problem is a difficulty on analyzing neutron absorption ability with lots of impurities. The vitrification form has lots of impurity. Thus, it is impossible to distinguish capture and scattering rate with conventional attenuation test. As a result, it is required to develop a new testing method of the neutron absorption ability for the absorber. In this study, a new testing method is proposed to verify neutron absorption ability of vitrified form.

To verify the estimation system of absorber performance, MCNP simulations were pursued with target materials which are the polyethylene, iron, vitrified form, and normal glass. MCNPX 2.61) with ENDF-VI and sab2002 cross section library was used for the analyses. The particle histories are decided that the all relative errors of the results are within 2%.

The results with thermal neutron fluxes in detector are given in Tables 3, 4, 5, and 6. Table 3 is the results of Steps 1 (C1) and 2 (C2) which are used to suppress the background.

Step 3 is results for the case using single target. It was evaluated that the normal and background suppressed fluxes with the proposed neutron absorber are 74.26 #·cm⁻²·sec⁻¹ and 13.15 #·cm⁻²·sec⁻¹, respectively. Using the results in Tables 3 and 4, absolute neutron absorption ability can be quantified. As shown in Table 4, the estimated flux using the glass absorber is much lower than normal glass. Therefore, it can be simply distinguish that the neutron absorber is well manufactured.

However, in the absorber based on vitrified form, lots of impurities can be included. To confirm the neutron absorption ability in the situation, Step 4 simulations were pursued. The results are given in Table 5. Using the results in Table 5, flux ratio was calculated using Equation 3, and the results are given in Table 6. In our sensitivity study, the fraction B0 in Table 6 is generally lowly evaluated than 100% when a material has high absorption cross section. Using the property, the absorption ability can be easily quantified with the proposed test system even though there are lots of impurities in an absorber.

In this study, to quantify the neutron absorption performance for neutron absorbers based on vitrification form, a performance test method was proposed. The proposed method can measure not only the direct thermal neutrons passing through materials, but also the scattered neutrons reflected to the materials. Therefore, it can efficiently quantify the neutron absorption performance for the materials which has lots of compositions. To verify the test system, MCNP simulations were performed using the proposed scheme. The results show that the neutron absorption performances can be effectively estimated using the proposed method comparing with the conventional neutron attenuation test. It is expected that the proposed method can contribute to increase the reliability of the performance test for newly developed neutron absorbers.